The electronic technology has recently made remarkable progress and has been successful in the reduction in size and weight of various devices. Along with the miniaturization and weight reduction of electronic devices, batteries that supply power to such devices are also required to be smaller and lighter. Nonaqueous electrolyte lithium secondary batteries are a type of batteries which can supply large amounts of energy with small volume and mass of the batteries. Further, nonaqueous electrolyte secondary batteries are proposed to be used as power sources for hybrid vehicles, electrical vehicles and the like, and some have come into actual use.
In electrodes of nonaqueous electrolyte secondary batteries, polyvinylidene fluoride (PVDF) is used as a binder (a binder resin).
PVDF is excellent in such properties as chemical resistance, weather resistance and contamination resistance, and exhibits excellent properties such as electrochemical stability, mechanical properties and slurry characteristics. However, PVDF shows weak adhesion with respect to metal foils used as collectors in nonaqueous electrolyte secondary batteries. To remedy this, various methods have been proposed in order to improve the adhesion of PVDF.
For example, vinylidene fluoride copolymers have been proposed which are obtained by graft polymerization of an acrylic monomer to a PVDF backbone by the application of radiation (see, for example, Patent Literature 1). Such vinylidene fluoride copolymers exhibit higher adhesion to metal foils than does PVDF. However, the fact that radiations are utilized in the production of such copolymers requires a special facility as well as a difficult control.
Other copolymers are also known, including fluorine-containing copolymers obtained by copolymerizing vinylidene fluoride with maleic acid anhydride and hydrolyzing the acid anhydride (see, for example, Patent Literature 2); vinylidene fluoride copolymers obtained by copolymerizing vinylidene fluoride with an unsaturated dibasic acid monoester such as monomethyl maleate (see, for example, Patent Literature 3); and vinylidene fluoride copolymers obtained by copolymerizing vinylidene fluoride with an acrylic acid compound (see, for example, Patent Literature 4).
However, these polymers are still unsatisfactory in terms of adhesion to metal foils.